All systems that include a combustion engine generate waste heat during operation which must somehow be rejected from the system. In some systems, such as for example high speed aircraft or space vehicles, it is difficult or impossible to reject the waste heat to ambient air. To overcome this problem, some systems employ elaborate cooling devices, such as water boilers, for waste heat rejection from the combustion engine and its loads. Such devices are often undesirable because of factors, such as added complexity, cost, system volume, and/or weight.
Another problem, unrelated to waste heat rejection, is fuel control in combustion engines that use an oxidizer or fuel that is stored in a liquid phase and which vaporizes as it travels to the combustor. For example, it is common for gas turbine auxiliary power units on high speed aircraft and space vehicles to utilize a gaseous oxidizer that is compressed and cooled and stored in an oxidizer supply tank in the form of a liquid for purposes of reducing the storage volume. One problem with such systems is that under certain conditions, such as at start-up, the oxidizer flow to the flow control valve and combustor may be a mixture of gas and liquid, and not entirely vaporized. This produces unacceptable variability in the mass flow rate of the oxidizer to the combustor because the mass flow rate of gaseous oxidizer is very different than the mass flow rate of liquid oxidizer when passing through the same control valve flow area. The variability this creates in the mixture ratio of the oxidizer and fuel can result in combustion temperatures that vary greatly, which can produce either flame out due to insufficient oxidizer or burn out due to too much oxidizer.